This invention relates to a toroidal whistle which when horn loaded provides uni-directional sound production with relatively high efficiency and which can be operated either by air or steam of relatively low pressure (e.g. 15 psig). Although not so limited, the whistle of the invention has utility on board ships, as a civil defense warning device, for acoustic stress testing of components for the aerospace industry and as coded signaling and/or warning devices.
At a frequency of 440 hertz, the whistle of the present invention produces a sound output of 135 decibels at 100 feet on axis. At a frequency of 220 hertz a whistle in accordance with the invention would be capable of producing a sound level up to 55 decibels at a distance up to 12 miles.
U.S. Pat. No. 4,429,656, issued Feb. 7, 1984 to the present applicant, discloses a toroidal closed chamber whistle capable of producing levels up to 125 decibels at 100 feet at a frequency of 440 hertz. The whistle of this patent provides a torus positioned on a hollow cylinder, a circular cover affixed to the top of the torus, an annular tapered lip descending perpendicularly from the cover in alignment with an air passage from the cylinder and surrounding the torus, the torus, lip and cover forming a toroidal chamber in which a sound wave is generated. The tapered lip and aligned air passage are thus arranged at the outer diameter of the toroidal chamber. The length of the toroidal sound chamber is three times the width thereof, and the outer diameter of the chamber is about 0.625 times the fundamental wave length of the generated sound. The whistle of this patent produces a bidirectional sound.
U.S. Pat. Nos. 48,921, issued July 25, 1865 to A. Fitts and 596,257, issued Dec. 28, 1897 to L. Bartlett, disclose early steam whistles typical of the prior art which required steam pressure of at least about 40 to 60 psig, and which provided cylindrical whistle bells each having a depending lip around the outer edge aligned with an annular slit through which steam passes under pressure, thereby generating sound.
German Pat. Nos. 84,935, published January 1896 and 523,008, published April 1931, disclose somewhat similar arrangements wherein a circular depending tapered lip surrounds a sound chamber, with the lip being aligned with a narrow annular slit through which steam under pressure passes. The lower portion of the whistle disclosed in German Pat. No. 94,935 has a cross-section which resembles that of a vortex whistle, a type which has less output than that of an ordinary cylindrical whistle.
Whistles produce maximum sound output when the radiating mouth area (between the tapered lip and narrow annular slit) is equal to the cross-sectional area of the chamber. Increasing the radiating mouth area beyond that of the chamber only increases the required operating pressure but not the sound output. A decrease in mouth area results in a loss of output, e.g. a mouth area half that of the chamber cross-section would result in an estimated loss of about 12 decibels. With the arrangement shown in the drawing of German Pat. No. 84,935, high output would thus be impossible.
Single chamber whistles of the type disclosed in the above early patents are limited in sound output by the capacity of the single chamber, are relatively inefficient (about 1% to 3%) and require high pressure steam or air. The output of such a whistle would be about 110 decibels at 100 feet, if a chamber length to chamber diameter ratio of 3:1 is used. An increase in diameter of the cylindrical chamber relative to its length does not significantly increase the output but would require more compressed steam or air. In an effort to improve output, two of the above-mentioned patents disclose the provision of two or more chambers combined into a single system. However, such a system cannot produce a directional output due to the small radiating area, and since the individual whistle chambers cannot be phase-locked to a single pitch, attempts to blow several whistles of the same pitch may actually result in reduced output. Whistles of different pitches would increase output but would result in a multi-pitch sound.
The whistle of the present invention is distinguishable from that of applicant's above-mentioned U.S. Pat. No. 4,429,656 in providing a substantially toroidal body secured to a hollow cylinder with a tapered lip on the inner wall of the toroidal chamber rather than on the outer wall. This has made possible an unexpected increase in efficiency of up to 25% by permitting the use of less air or steam under pressure to obtain equal power output. When using the same amount of air, a correspondingly greater power output is produced, and horn loading concentrates this output into a uni-directional sound beam of 60.degree. dispersion with up to 10 decibels greater output on axis. It will of course be understood that the whistle of the present invention can be adapted to full 360.degree. coverage if desired, by rotating the unit.
As indicated above, the whistle of U.S. Pat. No. 4,429,656 is bidirectional. The whistle of the present invention is substantially omni-directional without a horn, but is unidirectional when horn-loaded.
An increase in efficiency is obtained in the whistle of the invention due to the fact that blowing the toroidal chamber from the inner diameter thereof fully excites a radiating area (the area between the inner and outer diameters of a toroid) equal to that of U.S. Pat. No. 4,429,656 but uses only about 75% of the driving area (the area within the inner diameter of a torus) to do so, at the same steam or air pressure. For example a 20 inch diameter whistle of the present invention would have an annular slit of about 15 inches in diameter, whereas a 20 inch diameter whistle of U.S. Pat. No. 4,429,656 would have an annular slit of 20 inches in diameter since it is blown from the outer diameter thererof. In this connection it will of course be recognized that a conventional cylindrical whistle has only an outside edge and hence can be blown only from the outer diameter.